It is convenient to use complementary N- and P-channel power FET transistors to form pulse drivers for high capacitance lines, as shown in the prior art of FIGS. 2, 3, 5, and 6. Such drivers find application as line drivers for large-screen cold cathode and plasma displays, for example.
An example of prior art is illustrated in FIG. 2, in which the output 56 is a function of the gap between the supply voltage 57 and ground. Zener diode 53 bridges the voltage difference between supply 57 and ground, and is selected to give reliable turn-on voltage to transistors 54 or 55. In this example, the zener voltage of 53 might be chosen to be the supply voltage 57 less the turn-on voltage of 54, or about 10 Volts.
Several problems with this prior art occur as the design requires use of higher voltage at output 56. The first is that as voltage 57 varies, intentionally by design, the value of zener diode 53 must be adjusted to compensate.
To alleviate this, a prior art circuit illustrated in FIG. 3, replaces zener diode 53 with a capacitor, permitting an arbitrary selection of supply voltage 57 without the limitations of a fixed-voltage gap-spanning device such as a zener diode 53.
A positive input pulse 41 drives the gate of transistor 42, bridging the power supply 46 voltage gap via capacitor 44 to turn transistor 43 off. That is, the gates of transistors 42 and 43 follow each other exactly because of the coupling capacitor 44, although they may be, for example, 400 volts apart. If there is perhaps 2 to 5 volts of noise on power rail 48, transistor 43 will improperly be turned on.
A second issue with simple capacitive coupling is that long Input pulses at input 41 are differentiated through the R-C time constant of resistor 45 and capacitor 44, causing eventual loss of drive signal at transistor 43. The impact of this is a limitation on the duty cycle (on/off ratio) obtainable with capacitive coupling. This problem is corrected by the Schmidt Trigger memory characteristics of the present invention.
A third problem in capacitive coupling is the impact of noise. While capacitive coupling tends towards a low-cost solution to other problems, it behaves erratically when higher voltage operation is contemplated. Region A shown in FIG. 4 shows an example desired supply voltage 57, a clean unchanging value.
Section B shows AC ripple that might be found on a typical power source, and Section C shows some additional noise that might also be present on such a source. A realistic power source typically guarantees that ripple and noise will not exceed 5% or 10% of the desired supply voltage, of 250 Volts, in this case.
Presumably, a clean input signal is supplied at input 41 and is properly transferred to the gate of transistor 43, again without noise. The source of transistor 43 that is connected to rail 48 is wandering, following the noise on that rail. Whenever the noise exceeds the turn-on parameter for transistor 43, that transistor turns on. In fact, the noise on that rail is commonly 5% to 10% of the supply voltage, ensuring that this driver will not properly work at voltages much higher than 40 volts or so. The present invention circumvents this problem when operating at higher voltages.
That is, the undesired signals riding on the supply line 57 might be as much as 12 to 25 volts. This is well in excess of the voltage needed to turn transistors 54 or 43 on (or off), causing unreliable operation of the drivers at higher voltages. As the supply voltage increases, noise eventually exceeds the turn-on voltage for the FET transistors, causing this erratic operation with simple capacitive coupling. Where current sources are used to bridge the rail-to-rail voltage difference, increased supply voltage directly increases power dissipation.
FIG. 5 illustrates another example of prior art, the replacement of zener diode 53 with a circuit of current sources 20–23. These current sources 20–23 span the high voltage gap between the upper 33 and lower 34 supply rails, as supplied by battery 30. Field Effect Transistors (FET) 26 and 27 form a complementary pair whose gates are separated by approximately the full supply voltage of voltage source 30. Constant current sources 20 and 21 may be replaced by fixed resistors. The input pulse 25 is applied to FETs 26 and 29, and it is inverse to FET 28, bringing output 31 low. The resultant change in currents from sources 22 and 23 is sensed by comparator 24, which shuts off the upper FET 27. In similar manner, reversing pulse 25 reverses the balance of sources 22 and 23, and reverses the conductions through FETs 26 and 27.
The voltage dropped across sources 22 and 23 would be approximately the rail-rail drop supplied by battery 30. The power dissipated in those sources would be the product of their current and that voltage, an increasingly large dissipation as the voltage source 30 increases. When only a few such constant current switching circuits can be designed to withstand the required dissipation, the matter quickly becomes unwieldy when scores or hundreds of such circuits are required, such as in a high-voltage plasma or carbon nanotube display, or in driving other forms of relatively high capacitance loads. The sum of the I*V products makes the approach impractical.
Yet another example of prior art is given below in FIG. 6, a transformer-coupled driver. This overcomes most of the noise-related limitations of capacitive coupling, but brings with it several other limitations. An Input signal 61 is amplified by 62 and applied to the gates of transistors 65 and 66. In this simplified schematic, a positive pulse 61 will turn transistor 65 on and transistor 66 off, or visa versa, leading to an Output pulse 68 whose magnitude is that of power rail 67. The rail 67 voltage, 250 Volts, for example, is spanned by the isolated secondary windings of transformers 63 and 64, eliminating much of the noise problem of the circuit illustrated in FIG. 3. This is because the secondary windings isolate the gate drive signals for transistors 65 and 66 from the common-mode noise of the power rail 67.
The transformer solution has two problems: the transformers are relatively costly, and the pulse width and duty cycles are limited by the characteristics of the transformer magnetics. The pulse width of Input 61 is limited by the R-C time constant of the capacitively-coupled circuit of FIG. 3, and by the coercivity of the iron in the transformers of FIG. 6. The present invention resolves these problems.